1. Field of the Invention
This invention relates to electrical rotating machines connected to a common shaft and more particularly to a method of controlling a number of mechanically coupled electrical rotating machines such as wind turbines and ocean current turbines.
2. Description of the Prior Art
Many electric power-generating devices, such as wind turbines and ocean current turbines, benefit from economies of scale, yielding lower costs for generated electricity with larger power generation per unit. This increase in power is often accompanied by a reduction in rotational velocity of the power-input shaft, resulting in a large increase in torque. Because electric generators require rotational velocities tens to hundreds of times greater than the rotational velocity of the input shaft, a speed-increasing gearbox is often applied between the power-input shaft and the generator.
Dehlsen et al U.S. Pat. No. 6,304,002 discloses a Distributed Generation Drivetrain (DGD powertrain) for wind turbines and ocean current turbines which consists of a large, input power shaft-mounted, rotating bull-gear with stationary smaller powertrains mounted around its periphery, the shafts of which engage the bull gear by pinions. The gear teeth on the bull-gear rotate past the teeth on the pinions, causing the pinions to turn and deliver power to each smaller powertrain. Alternatively, powertrains are attached in a spindle around the perimeter of a main power input drive shaft, and rotate as the shaft rotates. The input drive shaft to each of the smaller powertrain gearboxes is fitted with a pinion. As the main power input shaft turns, the smaller powertrains, gearboxes and pinions rotate, moving the pinions around the interior of a stationary ring gear. Reduction and distribution of torque is similar to the rotating bull-gear powertrain. In the bull-gear configuration, each smaller powertrain is stationary, reducing stress caused by rotation.
In the above-identified U.S. Pat. No. 6,731,017 discloses an improved distributed powertrain that increases electric power generator density by locating a plurality of output shafts around a bull gear which is coupled to the main input shaft. A number of generators are each coupled to a respective one of the output shafts, such that each one of the output shafts drives a generator. A number of intermediate gears are located around a perimeter of the bull gear, alternating half of the intermediate gears on one side of the bull gear and half on an opposite side of the bull gear. Each one of the intermediate gears is pressure fitted to an intermediate pinion with teeth that engage the bull gear teeth. Each one of the output shafts has two pinions that engage two intermediate gears, one on one side of the bull gear and one on an opposite side of the bull gear.
The power and control system for the powertrain must take the mechanical output from the individual second-stage gearboxes and produce 3-phase electrical power to a utility line. To perform successfully, this control system must assure a uniform torque load distribution between generators, softly connect and disconnect with the utility line, seek maximum operating efficiency, monitor and provide protection for mechanical and electrical parameters operating out of specification, and accommodate input from external systems and operators.
Therefore, besides generators, the power conversion system requires an appropriate controller to manage these tasks. In patent application publication U.S. 2002/0014773 A1, filed Jul. 31, 2001, and published Feb. 7, 2002 (now U.S. Pat. No. 6,653,744) a control method is described that regulates torque experienced by each generator to assure that torques are balanced between generators at any given system load. The regulating includes controlling local voltage at each generator by a transformer configured as a reactor, in which coils of the transformers are wired in parallel and are actively modulated with a silicon controlled rectifier (SCR), a solid-state, switching device. Each generator is connected to a respective primary coil of a transformer and a respective secondary coil is connected to an SCR.
It is desirable that a Distributed Generation Drivetrain (DGD) powertrain having multiple generators be able to match the loads between generators evenly, avoiding “power hogging.” In the event of hogging, more power and more torque loads are passed through one pinion-bull gear interface than the others, endangering the gearing and the electrical equipment in the overloaded sub-powertrain.
It is also desirable for the control system for a wind (or ocean current) turbine utilizing the DGD powertrain to provide for variable speed operation when wind or water flow is sufficient to produce power, and the ability to provide for variable speed gust mitigation when peak power output of the turbine is reached.
Additionally, it is desirable for a DGD system having five or more generators to provide for very close load sharing between generators, and to allow for incremental engagement of generators in a way that maximizes system efficiency.